When you look at a photograph of a glowing cloud in space, you are looking at a nebula — one of the most important and most misunderstood objects in astronomy.

A nebula is a cloud of gas and dust in space, sometimes many light-years across. The gas is mostly hydrogen, mixed with helium and traces of heavier elements, and these clouds are the raw material of the universe.

Emission nebulae are the showpieces: glowing clouds where new stars are being born. Ultraviolet light from hot young stars makes the hydrogen gas fluoresce, producing the reds and pinks familiar from photographs. The Orion Nebula is the classic example.

Not every nebula glows on its own. Reflection nebulae shine by scattered starlight and tend to look blue, like the clouds around the Pleiades. Dark nebulae block the light behind them, appearing as inky patches against the Milky Way.

Two more types come from dying stars. A planetary nebula is the shell of gas a medium-sized star sheds at the end of its life. A supernova remnant is the expanding debris of a massive star that exploded, recycling heavy elements back into space.

Through a telescope, most nebulae look like faint grey smudges, not the vivid colors of photographs, because our night vision is nearly color-blind at low light levels. A nebula filter that blocks light pollution can dramatically improve the view.